Minimizing Drum-shaped Inaccuracy In High-speed Wire Electrical Discharge Machining After Skim Cuts

High-speed reciprocating wire-cut electrical discharge machining(HS-WEDM)has been widely used in aerospace,military industry,mold machining because of its remarkable characterstics of high performance-to-cost ratio and low operating costs.As far as its unique machining methods and excellent processing performance are concerned,it has becomed an irreplaceable processing technology in the field of mechanical manufacuring.In recent years,both the precision and surface quality of HS-WEDM have been significantly improved,thanks to the continuous development and perfection of computer numerical control(CNC)systems,pulse power generators,working fluids,and skim-cut processes.Currently,drum-shaped inaccuracy is one of the accuracy indicators,when the thickness of workpieces used in HS-WEDM is less than 40 mm,the drum-shaped inaccuracy that occurs after three cuts is less than 5?m.However,the drum-shaped inaccuracy dramatically increases as the thickness of the workpiece increases.For instance,when the thickness is in the range of 100~150mm,the drum-shaped inaccuracy after three cuts is usually approximately 15~25?m,making it difficult to trim cut the middle part of the workpiece.Therefore,the ability to use a skim-cut process in HS-WEDM for machining high-thickness workpieces is limited,especially in the processing of plastic moulds with high-thickness.In this paper,aiming at the problem that the drum-shaped inaccuracy increases dramatically in HSWEDM after skim cuts,the causes and control methods of the drum-shaped inaccuracy are systematically studied.The main work is sorted as follows:(1)A model of the positional changes of the wire electrode is established to analyze the causes of drum-shaped inaccuracy in HS-WEDM after skim cuts.And the wire bending deformation is simulated by the ANSYS software to provide a trend of the positional changes of the wire electrode when machining workpieces of different thicknesses are under different discharge energies.(2)The causes of the drum-shaped inaccuracy in HS-WEDM after skim cuts are studied in detail from the aspects of processing mechanism and technology.It is considered that the uneven material erosion of the upper,middle and lower parts of the workpiece is the main causes for the drum-shaped inaccuracy.This occurs for two reasons.On the one hand,the unbalanced forced on the wire electrode leads to the larger wire bending deformation,which makes the different discharge probabilities of the upper,middle and lower part of the workpiece.On the other hand,the characteristics of the HS-WEDM including the high resistance of the wire electrode and the uneven distribution of the working fluid,which leads to the different discharge energy at the different discharge points,result in different material removal of parts of the workpiece.(3)The drum-shaped inaccuracy is controlled in terms of the force acting on the wire electrode and the distribution of the discharge energy.On the one hand,to solve the unbalanced force acting on the wire electrode,novel measures were taken in this study.First,some measures have been taken to reduce the impulse from the effect of the discharge explosive force on the wire electrode.Second,the electrostatic force is introduced to counteract the partial deflection of the wire electrode.Finally,a closed-loop tension mechanism was designed to increase the wire tension,thereby improving the stability and rigidity of the wire electrode.On the other hand,the uneven distribution of the discharge energy can be improved by optimizing the position of the conduct block and the improvement of the woking fluid.(4)The technological experiments with the skim-cut were carried out for machining high-thickness workpieces in HS-WEDM.According to the result of tests,a unique trim-cut method is proposed.The final experimental results showed that the drum-shaped inaccuracy of a workpiece with a thickness of 150 mm was reduced to 10?m from 25?m after three cuts and the quality and uniformity of the machined surface were improved with the new trim-cut method and the improvements.